The milky-way and Andromeda galaxy are going to collide in about 7 billion years. Most of the stars in the galaxy will fly right past each other(a galaxy is mostly empty space). Lots of them will be thrown out into deep space. The super massive black hole in each galaxy will spiral inward towards each other creating a even larger super massive black hole. here are some videos about the impending collision

This one will give you a more in depth explanation
YouTube - Milky way vs. Andromeda
This one is just a graphic simulation
YouTube - Milky Way/Andromeda
420ultimatesmokage Reviewed by 420ultimatesmokage on . theoretically (galaxies colliding) what would happen if two galaxies collided.... if space is infinite (which and alot of others believe) then technically, the galaxies may not be stationary and moving themselves... so what would (could, or what do you think) happen if two of them collided? Rating: 5